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Other Analyses |
Division of Geological Sciences, California Institute of Technology, Pasadena, California 91109, U.S.A.
Department of Geology and Mineralogy, Parks Road, Oxford
Differences in physical properties between orogenic areas and adjacent inactive regions extend to depths of at least 200, and possibly 400 km, into the upper mantle; these differences are evident from the anomalous travel-times of Love waves traversing orogenic regions. At shallow depths, near the crust-mantle interface, such differences may be investigated by means of seismic refraction and some recent studies in the Alps and the United States are reviewed; it appears that many active orogenic areas are characterized by (1) greater than average crustal thicknesses, (2) higher than average compressional wave velocities at the base of the crust, (3) unusually low velocity contrasts at the Mohorovi
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discontinuity and (4) higher temperatures in the lower crust and upper mantle than at equivalent depths in adjacent inactive areas. The magnitudes of these effects are such that the lower crust in orogenic areas must differ, either in phase assemblage or in chemical composition, from the crust in adjacent areas. If the former is the case, the orogenic lower crust could be of intermediate or mafic composition in the granulite facies of regional metamorphism; in inactive regions the same chemical composition could be expressed by hydrous mineral assemblages, characterized by lower velocites and densities. This model is compatible with an origin for the orogenic root by crustal shortening. Alternatively, if the high velocities of the lower orogenic crust reflect differences in bulk chemical composition, this could occur through the addition of mantle-derived magmas as part of the orogenic process; estimates of the volume of such additions necessary to give the observed seismic velocities show that they are comparable with the apparent crustal thickening; this model is incompatible with significant amounts of crustal shortening but is compatible with either no over-all change in horizontal crustal dimensions, or with small amounts of crustal extension. The alternative models are discussed.
